Machine And Method For Producing A Fibrous Web

ABSTRACT

A process for producing a web ( 1 ) of paper, cardboard or other fibrous material using a device ( 28 ) for applying starch is characterised in that the dry content of the web ( 1 ) of fibrous material is of less than 70% while the starch becomes adhesive, and in that the web ( 1 ) of fibrous material is heated to a temperature which is sufficient for causing the starch to become adhesive.

This invention relates to a method for producing a paper web, a paperboard web or some other fibrous web using an apparatus for applying starch.

Described in DE 100 33 213 A1 are a method and an apparatus for the single- or double-sided application of a liquid to pasty medium, preferably made of starch, onto a material web, preferably made of paper or paperboard, in particular corrugated paperboard, whereby the material web is first provided on one or two sides with a coating and only then passes through a press nip. The disadvantage of this solution is that there is no closed web guidance upstream from the coating unit. Free draws and the still low strength of the fibrous web in the press section, in particular given a low mass per unit of area of less than 125 g/cm² for example, result increasingly in tears in the production process. In addition there is a risk of the applied material settling on a contact drying unit downstream from the apparatus.

Disclosed in WO 99/22067 are a method and an apparatus for applying a pasty coating medium onto a moving material web whereby the material web is first formed in a double mesh zone of a paper machine and a liquid or pasty coating medium is applied onto the material web, at least on one side, by means of an applicator. In this case another applicator for applying a liquid of pasty coating medium onto the material web is positioned downstream from a press element in the press section.

Known from WO 00/55423 are a method and an apparatus for treating a paper web or a paperboard web in a press section. A first applicator sprays a coating medium onto the web while said web is being passed over a suction roller by means of a felt. A second applicator sprays a coating medium onto the surface of a transfer belt from which the coating medium is then transferred to the web. Both applicators are arranged upstream from the first press nip.

On the known machine the paper is dried by supplying energy by means of heated cylinders, belts, in particular metal belts, or in non-contact mode by radiant drying or impingement drying.

During drying by means of drying cylinders the paper web enwraps the drying cylinder. In this case the paper web is usually supported by one or more skins. As a rule the skin lies on the side of the paper web facing away from the heat supply, meaning on the outer side of the paper web. It is also possible, however, for a skin to be used between the paper web and the cylinders on both sides of the paper web. In some cases, however, it is also possible to do without the use of a skin either fully or in part.

The supplied thermal heat causes a steam pressure gradient to arise between the paper web and its surroundings. As the result, the water evaporates and the paper web is dried. The skins used are permeable to air and steam; consequently the resulting steam can escape.

Furthermore, the starches used in the production of paper webs and paperboard webs are non-gelatinized starches which have to gelatinize in the further course of the paper production process, usually in the drying section.

Starch is the main secondary product of photosynthesis. It is the reserve polysaccharide of nearly all plants. Unlike free glucose, starch is osmotically ineffective and largely immobilized in the plant. The formation and storage of starch in plants takes place in the form of starch grains. Said grains display specific forms, sizes and layers for each plant, hence it is possible on the basis of these characteristics to determine the origin of the starch under the microscope.

Starches and modified starches are used in various stages of the paper and paperboard production process, for example as slip starch, for spraying, in glue presses or for paper coating. The starches are used not only as a binder but also to improve the production process and paper properties such as stiffness, strength, gloss, printing and others. Two thirds of the starches used are native starches, the rest are starches which have been modified by oxidation, cationizing or some other derivatizings.

In this case, use is made of, among other things, the pronounced water absorption capacity of starch combined with swelling of the starch grains. The swelling capacity depends greatly on the temperature; up to approx. 40° C. it is reversible. In the case of suspensions in water, the diameter of air-dried starch grains increases by 30 to 40%. With further heating of the suspension above a certain temperature specific to the plant species from which the starch grains originate, there occurs an irreversible intensive swelling. A temperature within a temperature range extending from 75 to 90° C. is referred to as the gelatinizing temperature. The viscosity of the starch suspension rises sharply upon reaching the gelatinizing temperature, and the starch grains lose their morphology during the gelatinizing.

For the gelatinizing there has to be a sufficiently large quantity of water available at a sufficiently high temperature. The rising speeds in the production of paper and paperboard mean that there is less and less time available for gelatinizing the starch. This can lead to the gelatinizing of the starch not being completed. As the result, the increase in strength expected from using the starch does not come about.

It is the object of the present invention to create a method in which the non-gelatinized starch used is fully gelatinized.

According to the invention this object is accomplished with a method of the type initially referred to in that during the gelatinizing of the starch the dry content of the fibrous web is less than 70%, and in that the fibrous web is heated to a temperature which is sufficient for the starch to gelatinize.

Through the targeted and controlled control of the temperature and the dwell time of the fibrous web within a range of the selected temperature, in particular at the beginning of the drying, the complete gelatinizing of the starch is achieved. Given a homogeneous distribution of the starch looking in the direction of the paper thickness, the temperature for the gelatinizing must be reached everywhere. Given an inhomogeneous distribution of the starch, it suffices for the temperature to be reached in the layers containing the starch.

Advantageous further aspects of the invention emerge from the subclaims, the description and the drawings.

Advantageously the water required for gelatinizing the starch is made available through the water content of the fibrous web.

The speed of the gelatinizing increases with the temperature. According to the invention, the temperature for the gelatinizing is raised through selective obstruction or prevention of the evaporation and vaporization above the value reached in accordance with the prior art.

The steam pressure gradient between the paper web and its surroundings must be so small that no noteworthy drying occurs in the paper web or in the layers in which the starch was added.

Such a steam pressure gradient is achieved in an advantageous embodiment of the method in that the surface of the fibrous web is covered during the gelatinizing by a skin with a high resistance to diffusion while heat is supplied simultaneously to the fibrous web.

Alternatively, provision is made for the surface of the fibrous web to be covered during the gelatinizing by a skin that is impermeable to steam and/or air while heat is supplied simultaneously to the fibrous web.

The gelatinizing is also supported if the surroundings of the fibrous web are created with a very high relative humidity, in particular through a low level of air exchange or through the use of at least one steam blower box.

It is an advantage for the gelatinizing to be performed downstream from the press section or at the beginning of the drying section. In this case it is an additional advantage for the fibrous web to be heated to the gelatinizing temperature in the mesh section or in the press section.

It is an advantage for the gelatinizing to begin at a dry content of the fibrous web of more than 15%. In this way the gelatinizing begins in the mesh section, continues in the press section, and takes place mainly in the drying section on account of the time required therefor.

The gelatinizing is triggered and supported by the input of heat. For this purpose, use is made of the contact of the fibrous web with heated surfaces, for example the heated surfaces of a drying cylinder, a heated belt preferably made of metal, or a heated mesh. Heat is also input by means of steam, for example in a steam blower box. Sufficient heating can also be achieved by means of radiation in the IR or NIR range, gas-heated or electrically heated radiators, microwaves, electroinductive heating or a combination of such heat sources.

Gelatinizing is even faster if the temperature is increased during the gelatinizing by external pressure applied to the fibrous web.

Said pressure can be built up or increased by meshes and/or belts which are placed under high tension by hydrostatic or pneumatic means, by a pressure hood or the combination of a pressure hood with a drying cylinder, or by a pulse drying unit, in particular a shoe press with a heated opposing roller with special pressure arrangement.

The steam pressure existing in the web, reduced by the pressure applied from the outside, is not allowed to exceed the intrinsic strength of the paper web, otherwise delamination of the paper web will occur. If the web is raised above boiling temperature under the ambient conditions, the pressure applied must be slowly reduced toward the end of the unit (shoe press, pulse drying unit, pressure hood with drying cylinder) such that the vaporization rate does not lead to delamination of the paper web.

The duration of the pressure build-up and the pressure reduction are also relevant. In particular it is important for there to be a gentle reduction of the pressure in the fibrous web in order to prevent its delamination. If a pulse drying unit is used in combination with a pressure hood, the cooling is not performed until at the end of the unit, in particular in combination with a preheated “cooled belt”.

The invention can be implemented on single-row as well as two-row drying sections.

The gelatinizing takes place in a temperature range between 50 and 150° C., preferably in a range between 60 and 130° C. In a preferred embodiment, a maximum temperature of between 60 and 150° C., preferably between 70 and 130° C., is reached during the gelatinizing process. The time available for the gelatinizing, meaning the time in which the temperature prevails or acts on the fibrous web from outside and which must be derived from the typical dwell time of the fibrous web in the drying section, amounts to between 5 and 80% of the total dwell time in the drying section, preferably between 5 und 50%.

For the gelatinizing on the machine for producing the fibrous web, use is made of non-gelatinized, ground or unground starch which is obtained from potatoes, tapioca roots, rise, maize, wheat or other types of grain.

The starch is supplied together with the fibrous suspension or dilution water as slip starch for example. Or the starch is applied to the fibrous web in spray form. Using coating units it is possible to apply the starch directly or indirectly. Spray starch, for example, is applied in the area of the mesh section between the layers of the fibrous web if the mesh section is suitable for producing a two-layer or multi-layer fibrous web. Spray starch can also be applied in the press section. Starch in liquid form is applied using applicators such as spray tubes, film presses or curtain coating units.

If several layers of different composition are fed by a headbox onto the mesh or between the meshes, then it is conceivable to feed the starch into a part of the layers. Starch can be used on single-layer paper or paperboard as well as on multi-layer paper or paperboard, whereby in the latter case it may be applied in only a part of the layers.

A multi-layer paper or paperboard is understood to be a paper or paperboard in which the layers are individually formed and subsequently couched together.

A particular advantage of the invention is that the selection of available starch types is greater, at least in part, than according to the prior art. In this case the costs of production can be reduced by selecting less expensive types of starch. Because non-gelatinized starches are added at the beginning, less energy is required than compared to a process in which the starch is added already gelatinized because gelatinized starch binds water which cannot be removed mechanically. The thermal energy supplied for the gelatinizing according to the invention can be used for the drying. Higher production speeds can be achieved due to the reduced drying requirement.

This invention relates also to a machine for producing a fibrous web using one of the previously described methods.

The machine is characterized according to the invention in that in the drying section there is at least one skin with a high diffusion resistance or one skin impermeable to steam or air, and in that the fibrous web can be passed through the drying section together with the skin, in particular underneath the skin, or the fibrous web can be passed through the drying section sandwiched between two skins impermeable to steam or air or with a high resistance to diffusion.

Similarly, the invention relates to a machine characterized in that it comprises a drying cylinder covered by a pressure hood, whereby the fibrous web can be passed through between the drying cylinder and the pressure hood together with at least one skin.

In this case the skin can be preferably heated.

A machine on which provision is made for at least one steam blower box on the upper and/or lower side of the fibrous web is also an advantage.

In addition or alternatively to this, provision can be made for the machine to comprise a pulse drying unit or a radiant heater.

Alternatively or in addition, the machine has at least one heatable pressure hood to support the gelatinizing. The at least one pressure hood advantageously comprises a multiplicity of chambers in which, per chamber, the pressure can be lowered gradually in steps in the web running direction of the fibrous web.

Another advantageous embodiment of the machine provides for it to comprise a press nip with a heated press roller and/or a heated press shoe and/or a heated skin carried along on the press shoe side.

The invention will be explained below in more detail with reference to the exemplary embodiments. In the drawing:

FIG. 1 shows a single-row drying section with steam-impermeable skins,

FIG. 2 shows a two-row drying section with steam-impermeable skins,

FIG. 3 shows a steam blower box acting on a fibrous web,

FIG. 4 shows a drying cylinder acting together with a pressure hood,

FIG. 5 shows a heating device for heating a fibrous web,

FIG. 6 shows a pressure hood with a heating device,

FIG. 7 shows a temperature curve belonging to the pressure hood,

FIG. 8 shows a press nip with a heated press roller, and

FIG. 9 shows a pressure curve belonging to the press nip.

After non-gelatinized starch has been supplied to a fibrous web 1 (FIG. 1) in the mesh or press section, said starch is gelatinized in a drying section 2. This requires sufficient free water to be available in the drying section 2.

To ensure a sufficiently high water content, the evaporation of the water in the drying section is selectively obstructed until the gelatinizing is achieved. This is done by the surface of the fibrous web 1 being covered by a skin 3 with a high resistance to diffusion or by a skin 3 impermeable to steam or air while heat is supplied simultaneously to the fibrous web 1 by the drying cylinder 4.

The evaporation of the water is prevented particularly effectively if a skin 5 is also available on the lower side of the fibrous web 1. Such skins 3, 5 can be used in a single-row drying section 2 that comprises not only the drying cylinders 4 but also deflecting rollers 6, as well as in a two-row drying section 7 (FIG. 2) that comprises only drying cylinders 4. In both cases the skins 3, 5 create an environment with little exchange of air. In the case of the single-row drying section 2 it is also possible for the deflecting rollers 6 to be used at the top and the drying cylinders 4 at the bottom.

The skins 3, 5 can be used preferably over only a part of the drying sections 2, 7 until it is assured that the previously incorporated or applied starch has been gelatinized.

In another embodiment of a drying apparatus, a pressure hood 9 is mounted on a drying cylinder 8 (FIG. 3). The fibrous web 1 is passed through between the drying cylinder 8 and the pressure hood 9 together with at least one skin 10. If the skin 10 is not sufficiently impermeable, use is made optionally of a second skin 11 which is passed over deflecting rollers 12 around the pressure hood 9 while the skin 10 is passed on together with the fibrous web 1. On account of the heating of the fibrous web 1 by the drying cylinder 8, a considerable pressure is also created in the region between said cylinder and the pressure hood 9 by the skin 10 and, optionally or alternatively, by the action of the pressurized medium in the pressure hood 9 such that the gelatinizing in the fibrous web 1 is accelerated. The medium in the pressure hood 9, for example water, is preferably not only pressurized but can also be heated in order to increase the temperature in the fibrous web 1.

It is also possible to use a steam blower box 12 (FIG. 4) fitted above the fibrous web 1 to supply the steam for conveying heat and liquid simultaneously into the fibrous web 1 in order to promote the gelatinizing. Because the steam is produced some distance away from the fibrous web 1, the partial pressure of the steam decreases toward the fibrous web 1 and the fibrous web 1 absorbs liquid and becomes hotter simultaneously.

In addition to the steam blower box 12 there can also be a steam blower box 13 underneath the fibrous web 1.

Heat can also conveyed into the fibrous web 1 by way of belts or skins 14, 15 (FIG. 5), in particular if they are made of metal. In addition or alternatively, the heat is conveyed into the fibrous web 1 by way of radiant heaters 16, 17. In this case too, the steam-impermeable belts 14, 15 prevent the steam caused by the additional heating from escaping, thus leading to a desirable increase in pressure with additional promotion of the gelatinizing.

A pressure hood 20 (FIG. 6), which is fitted above the fibrous web 1 guided by the skins 18, 19 in the direction of the arrow A, serves to generate an increased pressure p1, in particular due to simultaneous heating, in a first chamber 21 in the web running direction. Preferably there is another chamber 23, which is separated from the chamber 21 by a partition wall 22 and in which a lower pressure p2 (p2<p1) prevails. In this way the temperature T in the fibrous web 1 (FIG. 7), which is raised by the pressure p1 prevailing in the first chamber 19, can be lowered gradually over the length L of the pressure hood 20. Delamination or any other excessive loading of the fibrous web 1 with quality-impairing effects is avoided by the slow temperature reduction.

Like the pressure hood 20, a pressure hood 24 is also mounted on the lower side of the fibrous web 1. The pressure hood 24 also comprises two chambers 26, 27 which are separated from each other by a partition wall 25. There is a higher pressure in the first chamber 26 than in the second chamber 27.

The second chamber 27 is also used preferably for cooling, for example using a refrigerated fluid, in order to dissipate again, at least in part, the heat generated in the fibrous web 1 in the region of the first chambers 21, 26. It goes without saying that both the pressure hood 20 on the upper side and the pressure hood 24 on the lower side of the fibrous web 1 can have a multiplicity of chambers which are arranged one behind the other in the web running direction and in which the pressure of the first chambers 21 and 26 respectively can be gradually reduced.

In another embodiment of the invention, immediately after the starch is applied by an applicator 28 (FIG. 8) in the press section, the fibrous web 1 is passed together with the felt 32 through a press nip 31 which is formed by a press roller 29 and a shoe press roller 30. The shoe press roller 30 comprises a press shoe 32. To achieve a temperature at which the gelatinizing can take place, the press roller 29 is heated. Alternatively or in addition, the press shoe 33 and/or the skin carried along on the press shoe side is heated.

Inside the press nip with the length L (FIG. 9) between the press roller 29 and the press shoe 33 there arises a pressure p which increases to a maximum and drops again at the end of the press nip. 

1. Method for producing a paper web, paperboard web or other fibrous web (1) using an apparatus (28) for applying starch, wherein during the gelatinizing of the starch the dry content of the fibrous web (1) is less than 70%, and wherein the fibrous web (1) is heated to a temperature which is sufficient for the starch to gelatinize.
 2. Method according to claim 1, wherein the water required for gelatinizing the starch is made available through the water content of the fibrous web (1).
 3. Method according to claim 1, wherein the evaporation and vaporization of water contained in the fibrous web (1) is obstructed or prevented during the gelatinizing of the fibrous web (1).
 4. Method according to claim 1, wherein the surface of the fibrous web (1) is covered during the gelatinizing by a skin (3, 5; 10, 11; 14, 15; 18, 19; 32) with a high resistance to diffusion while heat is supplied simultaneously to the fibrous web (1).
 5. Method according to claim 1, wherein the surface of the fibrous web (1) is covered during the gelatinizing by a skin (3, 5; 10, 11; 14, 15; 18, 19; 32) which is impermeable to steam and/or air while heat is supplied simultaneously to the fibrous web (1).
 6. Method according to claim 3, wherein the surroundings of the fibrous web (1) are created with a very high relative humidity.
 7. Method according to claim 1, wherein the method is performed downstream from the press section or at the beginning of the drying section (2, 7).
 8. Method according to claim 1, wherein the fibrous web (1) is heated to the gelatinizing temperature in the mesh section or in the press section.
 9. Method according to claim 1, wherein the gelatinizing begins at a dry content of the fibrous web (1) of more than 15%.
 10. Method according to claim 1, wherein the gelatinizing is triggered and supported by the input of heat, whereby the fibrous web (1) is heated by contact with at least one of heated surfaces steam, or radiation.
 11. Method according to claim 1, wherein the temperature is increased during the gelatinizing by external pressure applied to the fibrous web (1).
 12. Method according to claim 11, wherein the external pressure is built up by at least one of meshes and/or belts (3, 5; 10, 11; 14, 15; 18, 19; 32), hydrostatic or pneumatic means, a drying cylinder (8) covered by a pressure hood (9), a pulse drying unit, and combinations thereof.
 13. Method according to claim 1, wherein the gelatinizing takes place in a temperature range between 50 and 150° C., preferably in a range between 60 and 130° C.
 14. Method according to claim 13, wherein a maximum temperature of between 60 and 150° C., preferably between 70 and 130° C., is reached during the gelatinizing process.
 15. Method according to claim 1, wherein the time available for the gelatinizing amounts to between 5 and 80% of the total dwell time in the drying section, preferably between 5 und 50%.
 16. Machine for producing a fibrous web (1) using a method according to claim 1, wherein in the drying section (2, 7) there is at least one of a skin (3, 5; 10, 11; 14, 15; 18, 19; 32) with a high diffusion resistance or a skin (3, 5; 10, 11; 14, 15; 18, 19; 32) impermeable to steam or air, and wherein the fibrous web (1) can be passed through the drying section (2, 7) together with the skin (3, 5; 10, 11; 14, 15; 18, 19; 32).
 17. Machine according to claim 16, wherein the machine comprises a drying cylinder (8) covered by a pressure hood (9), whereby the fibrous web (1) can be passed through between the drying cylinder (8) and the pressure hood (9) together with at least one skin (10).
 18. Machine according to claim 16, wherein the skin is heated.
 19. Machine according to claim 16, wherein the machine comprises at least one steam blower box (12, 13) on the upper and/or lower side of the fibrous web (1).
 20. Machine according to claim 16, wherein the machine comprises a pulse drying unit or a radiant heater (16, 17).
 21. Machine according to claim 16, wherein the machine comprises at least one heatable pressure hood (20, 24).
 22. Machine according to claim 21, wherein the at least one pressure hood (20,24) comprises a multiplicity of chambers (21, 23; 26, 27) in which, per chamber (21, 23; 26, 27), the pressure can be lowered gradually in steps in the web running direction of the fibrous web (1).
 23. Machine according to claim 16, wherein the machine comprises a press nip (31) with a heated press roller (29) and/or a heated press shoe (33) and/or a heated skin carried along on the press shoe side.
 24. Machine according to claim 16, wherein the fibrous web (1) can be passed through the drying section (2, 7) underneath the skin (3, 5; 10, 11; 14, 15; 18, 19; 32).
 25. Machine according to claim 16, wherein the fibrous web (1) can be passed through the drying section (2, 7) sandwiched between two skins (3, 5; 10, 11; 14, 15; 18, 19; 32) impermeable to steam or air or with a high resistance to diffusion.
 26. Method according to claim 6, wherein the surroundings of the fibrous web (1) are created through a low level of air exchange.
 27. Method according to claim 6, wherein the surroundings of the fibrous web (1) are created through use of at least one steam blower box (12, 13).
 28. Method according to claim 10, wherein the fibrous web (1) is heated by contact with at least one of the circumferential surface of a drying cylinder (4), a heated belt or a heated mesh.
 29. Method according to claim 10, wherein the fibrous web (1) is heated by steam from a steam blower box (12, 13).
 30. Method according to claim 10, wherein the fibrous web (1) is heated by radiation in the IR or NIR range.
 31. Method according to claim 10, wherein the fibrous web (1) is heated by gas-heated or electrically heated radiators (16, 17), microwaves, electroinductive heating or a combination of these heat sources.
 32. Method according to claim 1, wherein the gelatinizing takes place in a temperature range between 60 and 130° C.
 33. Method according to claim 13, wherein a maximum temperature of between 70 and 130° C. is reached during the gelatinizing process.
 34. Method according to claim 1, wherein the time available for the gelatinizing amounts to between 5 and 50% of the total dwell time in the drying section. 